It is known that exposure of human or animal tissue to ionising radiation will kill the cells thus exposed. This finds application in the treatment of pathological cells. In order to treat tumours deep within the body of the patient, the radiation must however penetrate the healthy tissue in order to irradiate and destroy the pathological cells. In conventional radiation therapy, large volumes of healthy tissue can thus be exposed to harmful doses of radiation, resulting in prolonged recovery periods for the patient. It is, therefore, desirable to design a device for treating a patient with ionising radiation and treatment protocols so as to expose the pathological tissue to a dose of radiation that will result in the death of these cells, whilst keeping the exposure of healthy tissue to a minimum.
Several methods have previously been employed to achieve the desired pathological cell-destroying exposure whilst keeping the exposure of healthy cells to a minimum. Many methods work by directing radiation at a tumour from a number of directions, either simultaneously from multiple sources or multiple exposures from a single source. The intensity of radiation emanating from each source is therefore less than would be required to destroy cells, but where the radiation beams from the multiple sources converge, the total intensity of radiation is sufficient to deliver a therapeutic dose.
The treatment may be spread over a number of days or weeks, to allow recovery of the healthy tissue. This should recover more quickly, as it received a lesser dose. Accordingly, repeated doses spaced over time will eventually take a greater toll on the pathological cells.
It is therefore important to deliver the radiation as accurately as possible to the pathological cells whilst minimising the dose to healthy tissue. Progress in this regard allows greater individual doses to be delivered in each treatment step, thereby reducing the total number of treatments required and, in fact, reducing the overall dose delivered to a patient. For example, a prescription of 35 instances of a 2 Gy dose might be replaced by 15 instances of a 3 Gy dose, a technique known as “hypofractionation”. Taken to its logical extreme, this might be replaced with a single 45 Gy dose if the dosage delivered to healthy tissue can be reduced significantly. This approach, referred to more typically as radiosurgery, will evidently offer advantages to the patient in that fewer treatments are required and there is no risk of inconsistent positioning between treatments.
Various methods have been proposed to reduce the dose the healthy tissue whilst maximising the dose to pathological tissue. The simplest is to direct the beam of radiation from a number of directions. Thus, at the co-incidence of the beams the dosage with be approximately ‘n’ times the dosage delivered to other areas, where ‘n’ is the number of directions employed.
Collimation can also be employed to limit the beam size to the minimum required to illuminate the pathological tissue. Multi-leaf collimators (MLCs) are known, such as that described in EP-A-0,314,214, and these are able to shape the beam to a desired outline.
In WO-A-02/069,349, we proposed a system whereby a beam of radiation was swept across the region of interest whilst its width was modulated. This offers the great advantage of an unlimited length to the treatment area.
In “rotational conformal” collimation, a radiation source is rotated around the patient and collimated with an MLC. The shape of the MLC collimation is varied with the angle of approach so that the width of the beam always conforms to the projected outline of the tumour as seen in the beam direction. This is useful for some shapes but deals poorly with concavities or re-entrant shapes.
IMAT techniques are described in U.S. Pat. No. 5,818,902. This develops the rotational conformal technique further by allowing repeated rotations around the patient. In this way, doses can be built up in the tumour area step-by step. To decide on the MLC shapes and directions, computational methods are used. Each voxel of the region of interest is assigned a “cost function”, which reflects the cost associated with a specific dose. Thus, for example, a voxel in the tumour area has a high cost associated with a low dose, whereas a voxel in a healthy area will be opposite. Some sensitive areas such as the spine and the digestive tract can be given cost functions that place a particularly high cost on doses above a certain critical level. Computational processes then seek to minimise the cost function by manipulating the delivery options. IMAT can provide exceptional dose distributions.
IMRT is similar in its computational principles to IMAT, but provides for a series of MLC-shaped beams from the same direction. Thus, the computational load is somewhat reduced.
Tomotherapy is a treatment technique described (for example) in ‘Planning Evaluation for complex lung cancer cases using Helical Tomotherapy’ T. Kron et al. Phys. Med. Biol. 49 (2004) 3675-3690. In this technique, a modulatable fan beam is produced from a source that is rotated around the patient in a helical fashion. The beam's intensity can be modulated by elements that slide into and out of the path of the fan beam across its width. The dose can be very conformal and the dose distributions achieved are impressive.